Method of joining a body of semiconductor material to a contact or support member

ABSTRACT

An aluminum coating is applied to a flat surface of a body of semiconductor material. The body having at least two zones or regions of different or opposite type conductivity exposed at the flat surface. A surface of a metal support or contact member which is to be joined to the aluminum coated surface of the semiconductor body is coated with a silicon-aluminum alloy. The two surfaces are then brought together and the surfaces heated to a temperature just above the silicon-aluminum eutectic whereby the body of semiconductor material is joined to support or contact.

United States Patent Raithel 51 May 30, 1972 METHOD OF JOINING A BODY OF SEMICONDUCTOR MATERIAL TO A CONTACT OR SUPPORT MEMBER [72] Inventor: Kurt Raithel, Uttenreuth, Germany [73] Assignee: Siemens Aktiengesellschaft, Berlin and Munich. Germany 22 Filed: on. 6, 1969 211 App]. No.: 864,121

3,330,030 7/1967 Broussard ..29/ 589 3,375,l43 3/1968 Garner et al. ...29/473.l X

3,382,568 5/1968 Kuiper..... .29/590 X 3,461,462 8/ 1969 Ruggiero ..29/492 3,537,174 11/1970 May ..29/590 X OTHER PUBLICATIONS Herbert Kroemer, Method of Making Ohmic Contacts to Silicon," RCA Technical Note No. 8, 8/57.

Primary Examiner.lohn F. Campbell Assistant Examiner-Ronald J. Shore Attorney-F. Shapoe and C. L. Menzemer [5 7 ABSTRACT An aluminum coating is applied to a flat surface of a body of semiconductor material. The body having at least two zones or regions of different or opposite type conductivity exposed at the flat surface. A surface of a metal support or contact member which is to be joined to the aluminum coated surface of the semiconductor body is coated with a silicon-aluminum alloy. The two surfaces are then brought together and the surfaces heated to a temperature just above the silicon-aluminum eutectic whereby the body of semiconductor material is joined to support or contact.

6 Claims, 4 Drawing Figures METHOD OF JOINING A BODY OF SEMICONDUCTOR MATERIAL TO A CONTACT R SUPPORT MEMBER BACKGROUND OF THE INVENTION 1. Field of the Invention I This invention is in the field of semiconductor devices.

2. Description of the Prior Art It is well known practice to apply a molybdenum support member to a flat side of a silicon disc or body, the side having on it only a single p-conducting zone. Usually the arrangement is such that a thin foil of aluminum metal is inserted between the silicon disc and the molybdenum support member and the composite is then heated to an alloying temperature which is appreciably higher than the eutectic temperature of silicon and aluminum.

This procedure is bound up with many difficulties, however, when it is required to apply a metal support member to a flat side of a silicon disc when the said side incorporates at least two zones of different conduction types. During the alloying process, a surface layer having a certain thickness melts on the fiat face of the silicon disc with the formation of a silicon-aluminum alloy melt. During the cooling down operation subsequent to the alloying process, part of the silicon in the mo]- ten surface layer again crystallizes out on the silicon disc. The recrystallization zone thus obtained in the silicon disc contains a relatively large amount of aluminum and is consequently pconducting. As a rule, this recrystallization zone on the flat side of the silicon disc does not entail any drawbacks, but may even be advantageous in cases where, for example, only a single p-conducting zone is present. However, if, in addition, there is present one or more n-conducting zones, the recrystallization zone will contain at least part of these n-conducting zones. This originally n-conducting part will often become pconducting. Thus, an undesired pn-junction will have been formed in the original purely n-conducting zone.

If the n-conducting zones which were originally on the flat side of the silicon disc were very thin, it is possible that they will be completely removed from the recrystallization zone after alloying on the metal support member, and become overdoped by p-conduction producing dopant.

An object of this invention is to provide a method or process for joining a surface of a body of semiconductor material having at least one p-type region and one n-type region exposed at the surface, to a support member without adversely affecting either the p-type or n-type region.

Other objects will, in part, be obvious and will, in part, appear hereinafter.

SUMMARY OF THE INVENTION In accordance with the present invention and attainment of the foregoing objects there is provided a process for affixing a support member to a flat surface of a body of semiconductor material, said surface having at least one p-type and at least one n-type region exposed thereon, said process comprising the flat side or surface of the body having zones of different conducting type regions with an aluminum coating, and providing a surface of the support member with a coating which consists of a silicon-aluminum alloy, the flat surface of the silicon disc or body then being brought into intimate contact with the mounting surface of the support member, after which the said silicon disc or body and support member being heated to a temperature which is at least equal to that of the silicon-aluminum eutectic, and at most to 600C. It is advantageous if a silicon layer is additionally applied on the initial silicon-aluminum alloy coated already applied to the support member.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the nature and objects of the invention, reference should be had to the following detailed description and drawings, in which:

FIG. 1 is a cross section through a semi-conducting element made in accordance with the teachings of this invention;

FIG. 2 is a plan view on a flat side of the silicon disc of the semiconducting element shown in FIG. 1;

FIG. 3 is a plan view of the other flat side of the silicon element; and

FIG. 4 shows a cross section through an alloying mold for producing the semiconducting component of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates a bilateral thyristor or semi-conductor switch which consists of a disc-shaped silicon disc or body 2 onto the under flat surface of which is attached a support member 3 consisting of tungsten, tantalum, molybdenum or base alloys thereof. The silicon body 2 comprises a sequence of three continuous zones 4, 5 and 6 which alternately have opposed types of conductivity. For example, the center zone 5 of n-type conductivity whereas the two other zones 4 and 6 are of p-type conductivity. In top flat surface 3 of silicon body 2, an n-type conductivity zone 7, which has the form of a half ring, is inlaid in p-type conductivity zone 4. The n-conducting zone 7 is provided with a contact electrode 8 which, as shown in FIG. 2, is likewise in the form of a half ring. An additional half ring contact electrode 10 is mounted on the same flat surface of p-conducting zone 4 in such a way that it forms, with contact electrode 8, a radially subdivided ring. Two small discshaped contact electrodes 11 and 12 are located in the gaps or cutouts 8a and 10a of the said radially divided rings. The small contact electrode 12 located in cutout 10a of contact electrode 10 has an n-conducting zone 13 located beneath it, which is set in p-conducting zone 4.

Two n-conducting zones 14 and 15 are recessed into p-conducting zone 6 on flat face 21 of the silicon body 2. As may be seen from FIG. 3, the n-conducting zone 14 is shaped in the form of a half ring or semicircle and is rotated through an angle of with respect to the n-conducting zone 7 on the top surface 3. The n-conducting zone 15 is in the form of a circular disc and is located at the center of the fiat surface 21 in the cutout 14a of zone 14.

The flat side 21 of silicon body 2, which is provided with zones 6, l4 and 15 is covered with an aluminum coating 9 which is not shown in FIG. 3. To this side is attached a discshaped molybdenum support member 30 which is provided, on its attachment surface 31 with a foil 16 consisting of an aluminum-silicon alloy.

The production of n-conducting zones 14 and 15 can be advantageously effected by means of a masked-off diffusion, of, for example, a phosphor. To this end, the silicon body 2 with its set of zones 4, 5 and 6 is heated in a water vapor atmosphere to about 900C. As a result, an oxide coating is formed on the surface of silicon body 2 which is impermeable to the phosphor. Apart from the places where zones 14 and 15 are to be, a coating of asphalt lacquer is applied to the oxide layer. Finally, silicon body 2 is etched with hydrofluoric acid so that the oxide coating is removed from the places which are not protected by the asphalt lacquer. After removing the lacquer coating, the phosphor is diffused in conventional manner into the silicon body 2 to form zones 14 and 15. After this, the remaining portions of the oxide layer are removed by etching in hydrofluoric acid. Subsequently, suitably shaped foils are alloyed in to form contact electrodes 8, 10, 1 1 and 12, as well as the recrystallization zones 7 and 13 on the other flat side of semiconductor body 2. The foils from which contact electrodes 8 and 12 are made contain, for example, antimony. Finally, aluminum is vapor deposited onto the flat side of the disc-shaped silicon body 2 which contains zones 14 and 15. The thickness of the said vapor-deposited aluminum coating should preferably lie between 10 and 40pm. The best thickness is 30pm.

When applying the aluminum coating to the flat side of the silicon disc 2, it is advantageous if the temperature of the said disc does not exceed that of the silicon-aluminum eutectic or about 577C. Preferably, the silicon disc should be held at room temperature or about 20C.

A coating 16, consisting of a silicon-aluminum alloy is applied to the mounting attaching surface 31 of the metal support member 30. The application can be effected, for example, in a high vacuum chamber in which support member 3 and an aluminum and silicon-containing tungsten boat are located. The tungsten boat is heated to a temperature of l,l to l,200C. and the support member 30 is heated to a temperature of 700 to 800C, and preferably maintained at 720C. The thickness of coating 16, which consists of a silicon aluminum alloy, is between 30 to 80am, and is preferably 40pm.

Instead of the above-mentioned alloy foil, a commercially available silicon aluminum alloy foil sold commercially under the trade name Silumin" may be applied to the mounting surface of the support member, using a graphite filled mold at a temperature of about 720C. This foil may have a thickness of 30 to 80 am, a preferred thickness being 40 m. Its composition is 87 percent by weight of aluminum and 13 percent by weight of silicon.

As illustrated in FIG. 4, silicon body 2, whose flat face 21 is provided with aluminum coating 9, is placed in an alloying mold 17 on the mounting surface of support member 30 which has been provided with the coating 16 consisting of a silicon aluminum alloy, the support member being centered by means of a graphite ring 18. Alloying mold 17 is then filled with graphite powder 19 and covered with lid 20. The mold 17 is then heated in an alloying furnace in a high vacuum to a temperature which is at least equal to that of the silicon-aluminum eutectic, 577C, and to 600C, at the most. A preferred temperature is 585C. As a result of this heat treatment, the aluminum coating on silicon body 2 combines with the silicon aluminum alloy coating on support member 30. In the process, practically none of the silicon constituting silicon body 2 is melted, and, on cooling, recrystallizes, so that the n-type doping of zones 14 and is retained and no undesirable pn-junction is formed.

During the heating process in the alloying furnace, it is possible for a silicide of the meta] constituting the support member 30 to form, whereby silicon from the silicon-aluminum coating on support member 30 is used up. In the case of a molybdenum support member, molybdenum silicide (MoSi can form, for example. In order to prevent the silicon of silicon body 2 from melting to compensate for the silicon used up in the formation of the silicide, it may be advantageous, prior to attaching support member 30 to silicon body 2, to vapor deposit additional layer of silicon onto the silicon aluminum alloy coating 16. This can also be effected in a high vacuum chamber. The thickness of this additional silicon coating can be between 0.5 and 2 pm. A preferred thickness is 1 am.

It is advantageous to provide the n-conducting zones 14 and 15 on the flat side of silicon body 2 with a coating of a donor material prior to applying aluminum coating 9. This donor coating can have a thickness of l to 2 am and is preferably vapor deposited on zones 14 and 15. As a result of this, the danger of an excessive doping of parts of zones 14 and 15 by aluminum is still further reduced. Antimony is especially suitable as a donor material because it is more easily soluble in silicon than aluminum.

I claim as my invention:

1. A method for joining a surface of a body of semiconductor material to a support member, the surface of the body of semiconductor material having at least one p-type region and at least one n-type region exposed thereat, comprising; disposing an aluminum coating on the entire surface of the body of semiconductor material, disposing a coating consisting of a silicon aluminum alloy on one surface of a metal support member, disposing the body of semiconductor material on the support member, the aluminum coating on the surface of the body being in direct contact with the silicon aluminum coating on the support member, and heating the body and support member to a temperature ranging from the eutectic temperature of the silicon aluminum alloy to 600C whereby the body is joined to the support member.

2. The method of claim 1 in which a second coating consisting of silicon is disposed over the silicon aluminum alloy coating prior to disposing the body on the support member.

3. The method of claim 1 in which the n-type region exposed on the surface of the body of semiconductor material has a coating consisting of an n-type doping material disposed thereon prior to disposing the aluminum coating on the entire surface of the body.

4. The method of claim 1 in which the silicon-alumin um alloy consists of, by weight, 87 percent aluminum and 13 percent silicon.

5. The method of claim 1 in which the aluminum coating has a thickness of from 10 to 40 pt m and the silicon-aluminum coating has a thickness of from 30 to pm.

6. The method of claim 3 in which the coating of n-type doping material has A thickness of from I to 2 ,um. 

2. The method of claim 1 in which a second coating consisting of silicon is disposed over the silicon aluminum alloy coating prior to disposing the body on the support member.
 3. The method of claim 1 in which the n-type region exposed on the surface of the body of semiconductor material has a coating consisting of an n-type doping material disposed thereon prior to disposing the aluminum coating on the entire surface of the body.
 4. The method of claim 1 in which the silicon-aluminum alloy consists of, by weight, 87 percent aluminum and 13 percent silicon.
 5. The method of claim 1 in which the aluminum coating has a thickness of from 10 to 40 Mu m and the silicon-aluminum coating has a thickness of from 30 to 80 Mu m.
 6. The method of claim 3 in which the coating of n-type doping material has A thickness of from 1 to 2 Mu m. 